How To Build Good Contact Mics

December 22, 2025 by 1 Comment

We’re most familiar with sound as vibrations that travel through the atmosphere around us. However, sound can also travel through objects, too! If you want to pick it up, you’d do well to start with a contact mic. Thankfully, [The Sound of Machines] has a great primer on how to build one yourself. Check out the video below.

The key to the contact mic is the piezo disc. It’s an element that leverages the piezoelectric effect, converting physical vibration directly into an electrical signal. You can get them in various sizes; smaller ones fit into tight spaces, while larger ones perform better across a wider frequency range.

[The Sound of Machines] explains how to take these simple piezo discs and solder them up with connectors and shielded wire to make them into practical microphones you can use in the field. The video goes down to the bare basics, so even if you’re totally new to electronics, you should be able to follow along. It also covers how to switch up the design to use two piezo discs to deliver a balanced signal over an XLR connector, which can significantly reduce noise.

There’s even a quick exploration of creative techniques, such as building contact mics with things like bendable arms or suction cups to make them easier to mount wherever you need them. A follow-up explores the benefits of active amplification. The demos in the video are great, too. We hear the sound of contact mics immersed in boiling water, pressed up against cracking spaghetti, and even dunked in a pool. It’s all top stuff.

These contact mics are great for all kinds of stuff, from recording foley sounds to building reverb machines out of trash cans and lamps.

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Lichtenberg Lightning In A Bottle, Thanks To The Magic Of Particle Accelerators

December 22, 2025 by 5 Comments

You’ve probably seen Lichtenberg figures before, those lightning-like traces left by high-voltage discharge. The safe way to create them is using an electron beam to embed charge inside an acrylic block, and then shake them loose with a short, sharp tap. The usual technique makes for a great, flat splay of “lightning” that looks great in a rectangular prism or cube on your desk. [Electron Impressions] was getting bored with that, though, and wanted to do something unique — they wanted to capture lightning in a bottle, with a cylindrical-shaped Lichtenberg figure.  The result is in the video below.

They’re still using the kill-you-in-milliseconds linear accelerator that makes for such lovely flat figures, but they need to rotate the cylinder to uniformly deposit charge around its axis. That sounds easy, but remember this is a high-energy electron beam that’s not going to play nice with any electrical components that are put through to drive the spinning.

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3D printed Origami mechanism

Origami On Another Level With 3D Printing

December 21, 2025 by 1 Comment

Origami has become known as a miracle technique for designers. Elegant compliant mechanisms can leverage the material properties of a single geometry in ways that are sometimes stronger than those of more complicated designs. However, we don’t generally see origami used directly in 3D printed parts. [matthew lim] decided to explore this uncharted realm with various clever designs. You can check out the video below.

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The Unusual Pi Boot Process Explained

December 21, 2025 by 6 Comments

If you’ve ever experimented with a microprocessor at the bare metal level, you’ll know that when it starts up, it will look at its program memory for something to do. On an old 8-bit machine, that program memory was usually an EPROM at the start of its address space, while on a PC, it would be the BIOS or UEFI firmware. This takes care of initialising the environment in both hardware and software, and then loading the program, OS, or whatever the processor does. The Raspberry Pi, though, isn’t like that, and [Patrick McCanna] is here to tell us why.

The Pi eschews bringing up its ARM core first. Instead, it has a GPU firmware that brings up the GPU. It’s this part of the chip that then initialises all peripherals and memory. Only then does it activate the ARM part of the chip. As he explains, this is because the original Pi chip, the BCM2835, is a set-top-box chip. It’s not an application processor at all, but a late-2000s GPU that happened to have an ARM core on a small part of its die, so the GPU wakes first, not the CPU. Even though the latest versions of the Pi have much more powerful Broadcom chips, this legacy of their ancestor remains. For most of us using the board it doesn’t matter much, but it’s interesting to know.

Fancy trying bare metal Pi programming? Give it a go. We’ve seen some practical projects that start at that level.

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Hackaday Links: December 21, 2025

December 21, 2025 by 4 Comments

It’s amazing how fragile our digital lives can be, and how quickly they can fall to pieces. Case in point: the digital dilemma that Paris Buttfield-Addison found himself in last week, which denied him access to 20 years of photographs, messages, documents, and general access to the Apple ecosystem. According to Paris, the whole thing started when he tried to redeem a $500 Apple gift card in exchange for 6 TB of iCloud storage. The gift card purchase didn’t go through, and shortly thereafter, the account was locked, effectively bricking his $30,000 collection of iGadgets and rendering his massive trove of iCloud data inaccessible. Decades of loyalty to the Apple ecosystem, gone in a heartbeat.

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Pause Print, Add Hardware, And Enjoy Strength

December 21, 2025 by 10 Comments

3D Printing is great, but it is pretty much the worst way to make any given part– except that every other technique you could use to make that part is too slow and/or expensive, making the 3D print the best option. If only the prints were stiffer, stronger, more durable! [JanTech Engineering] feels your plight and has been hacking away with the M601 command to try embedding different sorts of hardware into his prints for up to 10x greater strength, as seen in the video embedded below.

It’s kind of a no-brainer, isn’t it? If the plastic is the weak point, maybe we could reinforce the plastic. Most concrete you see these days has rebar in it, and fiber-reinforced plastic is the only way most people will use resin for structural applications. So, how about FDM? Our printers have that handy M601 “pause print” command built in. By creatively building voids into your parts that you can add stronger materials, you get the best of all possible worlds: the exact 3D printed shape you wanted, plus the stiffness of, say, a pulltruded carbon-fiber rod.

[JanTech] examines several possible inserts, including the aforementioned carbon rods. He takes a second look at urethane foam, which we recently examined, and compares it with less-crushable sand, which might be a good choice when strength-to-weight isn’t an issue. He doesn’t try concrete mix, but we’ve seen that before, too. Various metal shapes are suggested — there are all sorts of brackets and bolts and baubles that can fit into your prints depending on their size — but the carbon rods do come out ahead on strength-to-weight, to nobody’s surprise.

You could do a forged carbon part with a printed mold to get that carbon stiffness, sure, but that’s more work, and you’ve got to handle epoxy resins that some of us have become sensitized to. Carbon rods and tubes are cheap and safer to work with, though be careful cutting them.

Finally, he tries machining custom metal insets with his CNC machine. It’s an interesting technique that’s hugely customizable, but it does require you to have a decent CNC available, and, at that point, you might want to just machine the part. Still, it’s an interesting hybrid technique we haven’t seen before.

Shoving stuff into 3D-printed plastic to make it a better composite object is a great idea and a time-honored tradition. What do you put into your prints? We’d love to know, and so would [Jan]. Leave a comment and let us know.

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Why Chopped Carbon Fiber In FDM Prints Is A Contaminant

December 21, 2025 by 28 Comments

A lot of claims have been made about the purported benefits of adding chopped carbon fiber to FDM filaments, but how many of these claims are actually true? In the case of PLA at least, the [I built a thing] channel on YouTube makes a convincing case that for PLA filament, the presence of chopped CF can be considered a contaminant that weakens the part.

Using the facilities of the University of Basel for its advanced imaging gear, the PLA-CF parts were subjected to both scanning electron microscope (SEM) and Micro CT imaging. The SEM images were performed on the fracture surfaces of parts that were snapped to see what this revealed about the internal structure. From this, it becomes apparent that the chopped fibers distribute themselves both inside and between the layers, with no significant adherence between the PLA polymer and the CF. There is also evidence for voids created by the presence of the CF.

To confirm this, an intact PLA-CF print was scanned using a Micro CT scanner over 13 hours. This confirmed the SEM findings, in that the voids were clearly visible, as was the lack of integration of the CF into the polymer. This latter point shouldn’t be surprising, as the thermal coefficient of PLA is much higher than that of the roughly zero-to-negative of CF. This translates into a cooling PLA part shrinking around the CF, thus creating the voids.

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